The elapsed time taken by a signal travelling from the Earth to a spacecraft or other celestial body, then immediately transmitted or reflected back to the original transmission point. This is about equal to twice the one-way light time, but it varies because the motions of the Earth and space objects create different travelling times each way. RTLT to the Sun is about 17 minutes, and it was about 23 hours to the Voyager 1 probe in October 2001.